‘Superfog' not to be taken lightly, expert says

Aerial view of Interstate 75 in Gainesville, Fla. where according to Florida Highway Patrol at least 9 people have died as a result of multiple crashes Sunday January 29, 2012 involving 4 commercial vehicles and at least 10 passenger vehicles. The majority of the accidents happened in an area adjacent to where a brush fire was burning and producing heavy smoke.

Published: Sunday, January 27, 2013 at 5:15 p.m.

Last Modified: Sunday, January 27, 2013 at 9:53 p.m.

The monster that formed over Paynes Prairie on Jan. 29, 2012, and led to what is believed to be the deadliest set of accidents in Florida history wasn’t merely fog or smoke or a combination of the two.

It was a unique phenomenon that can arise when the conditions are ripe, and it could kill again.

Meteorologist Gary Achtemeier with the U.S. Forest Service knows it well. He had named it “superfog” and warns it is not to be taken lightly.

“There is only one course of action for a motorist encountering superfog, and it is not to drive. I liken it to a bridge collapse,” Achtemeier said. “It has to be stressed that it is a unique phenomenon and is extremely dangerous.”

Achtemeier is a research meteorologist at the Forest Service’s Center for Forest Disturbance Science in Athens, Ga., and has studied the conditions that brew together to create superfog for more than a decade.

One year ago on Tuesday, it materialized at around 4 a.m. on the south end of Interstate 75 where the highway crosses Paynes Prairie.

There, the remnants of a smoldering brush fire combined with a layer of fog.

Drivers described instantly having “a white blanket’’ over their windshields. Some stopped in their tracks in the northbound and southbound lanes, spawning a series of collisions that killed 11, sent another two dozen to hospitals and caused minor injuries to nearly two dozen others.

Superfog is rare. But it is also like the conundrum of the tree falling in the woods and no one hearing it — superfog may form more often than anyone knows because scientists generally know about it only when it affects roads.

It is exceptionally dense, with visibility measured in inches, Achtemeir said.

Superfog forms when the right mixture of two humid air masses with widely differing temperatures occurs.

The masses mix and create the release of moisture in the fog. The variable condition is the temperature and humidity of the ambient air. The cooler the temperature and the higher the humidity in the ambient air, the more likely that the conditions for superfog will be met.

It is most likely to form in the late night and early morning when the sun is rising or setting and impacting the temperatures.

Temperature and humidity are affected by fire, Achtemeier said. A smoldering brush fire in a low-lying, moist area, such as Paynes Prairie, will create heat and moisture. But that heat is not enough to create turbulence that would blow the smoke higher into the sky, as would heat from a flaming fire.

So the heat and moisture stay low. When the right combination of temperatures, humidity and smoke particles is reached, superfog is born in a localized area.

“There is a narrow range of conditions under which superfog can move and survive. It is a smoke plume with smoke particulates and the warm, moist air in the smoke from smoldering fires. Once that mixes with the ambient conditions, you either do or don’t have superfog,” Achtemeier said.

“In the Gainesville area, 35 to 55 degrees would be the cool temperature and relative humidity of above 80 percent. For the smoke, you would need temperatures of well over 100 degrees and humid smoke. You can see why this is rare in nature apart from fire. You very seldom get those kinds of conditions in nature.”

Achtemeier is trying to develop a superfog index for the Southeast — superfog is most prevalent in coastal states from South Carolina to Louisiana. Ideally, the index will be able to predict superfog based on temperature and humidity levels given other conditions such as smoldering fire and wind direction.

<p>The monster that formed over Paynes Prairie on Jan. 29, 2012, and led to what is believed to be the deadliest set of accidents in Florida history wasn't merely fog or smoke or a combination of the two.</p><p>It was a unique phenomenon that can arise when the conditions are ripe, and it could kill again.</p><p>Meteorologist Gary Achtemeier with the U.S. Forest Service knows it well. He had named it “superfog” and warns it is not to be taken lightly.</p><p>“There is only one course of action for a motorist encountering superfog, and it is not to drive. I liken it to a bridge collapse,” Achtemeier said. “It has to be stressed that it is a unique phenomenon and is extremely dangerous.”</p><p>Achtemeier is a research meteorologist at the Forest Service's Center for Forest Disturbance Science in Athens, Ga., and has studied the conditions that brew together to create superfog for more than a decade.</p><p>One year ago on Tuesday, it materialized at around 4 a.m. on the south end of Interstate 75 where the highway crosses Paynes Prairie.</p><p>There, the remnants of a smoldering brush fire combined with a layer of fog.</p><p>Drivers described instantly having “a white blanket'' over their windshields. Some stopped in their tracks in the northbound and southbound lanes, spawning a series of collisions that killed 11, sent another two dozen to hospitals and caused minor injuries to nearly two dozen others.</p><p>Superfog is rare. But it is also like the conundrum of the tree falling in the woods and no one hearing it — superfog may form more often than anyone knows because scientists generally know about it only when it affects roads.</p><p>It is exceptionally dense, with visibility measured in inches, Achtemeir said.</p><p>Superfog forms when the right mixture of two humid air masses with widely differing temperatures occurs.</p><p>The masses mix and create the release of moisture in the fog. The variable condition is the temperature and humidity of the ambient air. The cooler the temperature and the higher the humidity in the ambient air, the more likely that the conditions for superfog will be met.</p><p>It is most likely to form in the late night and early morning when the sun is rising or setting and impacting the temperatures.</p><p>Temperature and humidity are affected by fire, Achtemeier said. A smoldering brush fire in a low-lying, moist area, such as Paynes Prairie, will create heat and moisture. But that heat is not enough to create turbulence that would blow the smoke higher into the sky, as would heat from a flaming fire.</p><p>So the heat and moisture stay low. When the right combination of temperatures, humidity and smoke particles is reached, superfog is born in a localized area.</p><p>“There is a narrow range of conditions under which superfog can move and survive. It is a smoke plume with smoke particulates and the warm, moist air in the smoke from smoldering fires. Once that mixes with the ambient conditions, you either do or don't have superfog,” Achtemeier said.</p><p>“In the Gainesville area, 35 to 55 degrees would be the cool temperature and relative humidity of above 80 percent. For the smoke, you would need temperatures of well over 100 degrees and humid smoke. You can see why this is rare in nature apart from fire. You very seldom get those kinds of conditions in nature.”</p><p>Achtemeier is trying to develop a superfog index for the Southeast — superfog is most prevalent in coastal states from South Carolina to Louisiana. Ideally, the index will be able to predict superfog based on temperature and humidity levels given other conditions such as smoldering fire and wind direction.</p>